A typical wastewater treatment facility accepts input wastewater containing solid and dissolved waste matter. The solids and dissolved matter are removed, and the water is purified prior to discharge into the environment. The waste solids are commonly referred to as sludge or biosolids. Biosolids contain viral and bacterial pathogens, organic and inorganic contaminates. Thus biosolids pose a safety and health risk to humans and the environment. Biosolids have historically been disposed of by incineration, landfilling, or land application. Generally over half of wastewater facilities' operating costs are associated with the handling and disposal of biosolids.
An alternative to landfill disposal and/or incineration is the production of environmentally safe biosolids, including Class A biosolids. Biosolids purified to United States Environmental Protection Agency's (EPA's) Class A standards result in the virtual elimination of pathogens. Class A biosolids are safe enough to be utilized as fertilizer on agricultural land and possess almost no application restrictions.
Although meeting Class A standards permits the recycling of biosolids and eliminates costs associated with incineration or transfer to landfill, conventional processes for producing Class A pathogen levels are very costly. Class A biosolids require a significant amount of energy to process, and are thus associated with extremely high fuel demands. There exists an economic disincentive for Class A biosolid production, which in turn has hampered its widespread adoption.
Anaerobic digestion has been one of the most widely used processes for the stabilization of primary and secondary sludges produced at municipal wastewater treatment facilities, but the majority of applications of anaerobic digestion to wastewater sludges have been in the mesophilic temperature range, from 35° C. to 40° C. (95° F. to 104° F.). Anaerobic sludge digestion in the thermophilic temperature range, necessary for producing Class A biosolids, is performed at 45° C. to 65° C. (113° F. to 149° F.) but has been practiced to only a limited extent. The limited use of anaerobic digestion at higher thermophilic temperatures is due to the greatly increased energy demands and high fuel (e.g., natural gas) costs.
The advantages of thermophilic digestion include increased stabilization and methane production rates, and improved sludge dewatering properties. Furthermore, thermophillic anaerobic digestion coupled with a mesophilic digestion has improved pathogen destruction and meets the pathogen quality requirements for EPA's Class A biosolid certification. However, there exists a need for cost-efficient Class A biosolid processing.
Anaerobic digestion processes produce biogas byproducts such as CO2, nitrogen, and methane. Generally, this renewable source of energy is flared as a gas byproduct. Advantageously, the significant amounts of methane should be converted to usable heat and electricity. There exists a need for a cost-efficient method for harnessing the energy potential inherent in wastewater treatment process and utilizing said energy to sustain the wastewater treatment process.